Power coatings based on branched oligoesters and non-emissive uretdione polyisocyanates

ABSTRACT

The present invention relates to powder coating compositions which can be cured at low temperatures. The powder coating composition of the invention include a unique combination of a branched oligoester polyol and uretdione crosslinking agent which when cured results in a coating binder with desirable hardness, flexibility, solvent resistance, corrosion resistance, weatherability and gloss.

The present invention relates to powder coating compositions which canbe cured at low temperatures either with or without the use of aurethane catalyst. More particularly, the present invention relates tobranched hydroxyl terminated oligoesters which when crosslinked provideimproved performance properties at low curing temperatures and which donot release blocking agents from crosslinker into the environment.

BACKGROUND OF THE INVENTION

Thermosetting powder coating compositions are well known in the art andare widely used as coatings for electric appliances, bicycles, gardenfurniture, accessories for the automotive industry, general metal partsand the like. Thermosetting powders consist of a mixture of a primaryresin and one or more crosslinkers, often called hardeners or curingagents. The general approach associated with powder coating technologyis to formulate a coating from solid components, mix them, dispersepigments (and other insoluble components) in a matrix of the majorbinder components, and pulverize the formulation into a powder. In sofar as possible, each particle contains all of the ingredients in theformulation. The powder is applied to the substrate, usually but notlimited to a metal, and fused to a continuous film by baking.

Compositions which include organic polyhydroxy compounds and blocked toinclude internal or self-blocked polyisocyanates and which are solid atroom temperature, are important binding agents for thermallycross-linkable powder coatings (see for example, U.S. Pat. Nos.3,857,818 and 4,375,539). Common to these systems is the disadvantagethat, during thermal cross-linking, the compounds used as blockingagents, excluding self/internal blocked agents, are split off and escapeinto the environment. Therefore, during cure and crosslinking specialprecautions must be taken to purify the waste air and/or to recover theblocking agent for reasons of ecology and work hygiene.

The elimination of emissions from the curing of powder coatings has beenattempted with the use of blocking-agent-free,uretdione-group-containing polyurethane (PUR) powder coating hardeners.In these compositions cross-linking takes place with thermal cleaving ofthe uretdione groups. (See for example U.S. Pat. Nos. 5,621,064 and4,413,079). Typically, films produced with these types of uretdionecrosslinkers, however, do not have optimal film properties such ashardness, flexibility, solvent resistance, corrosion resistance,weatherability and gloss.

Another problem with powder coating compositions is that they frequentlyhave low glass transition temperatures (T_(g)) and will agglomerate orsinter when stored at elevated temperatures for a prolonged duration oftime. This phenomena causes an application problem when the powdercoating composition taken from storage is agglomerated and requiresremilling, which may or may not permit application of a powder coatinghaving a suitable particle size.

It is an object of the invention to provide a powder coating compositionwhich has a relatively high glass transition temperature and that willresist agglomeration during storage.

It is an object of the invention to provide a powder coating compositionwhich will maximize film properties such as hardness, flexibility,solvent resistance, corrosion resistance, weatherability and gloss, yetalso provide a coating composition with a relatively high glasstransition temperature.

It is another object of the invention to provide a powdered coatingcomposition which can be cured at temperatures as low as about 160° C.without the use of an effective amount of urethane catalyst or attemperatures less than about 160° C. with the use of an effective amountof urethane catalyst such as 1,5-diazabicyclo(4.3.0)non-5-ene,1,8-diazabicyclo(5.4.0)undec-7-ene, dibutyltin dilaurate, butanestannoic acid, dibutyltin oxide, and others known in the art.

It is yet another object of the invention to provide a thermosettingpowder coating composition that includes crosslinker that does notrelease a blocking agent into the environment upon curing.

It is another object of the invention to provide a powder coatingcomposition with OT bend performance, and accelerated cure schedules attemperatures greater than about 160° C. which are typical requirementsfor coil coating applications.

It is another object of the invention to provide a powder coatingcomposition with a desirable melt viscosity.

Other objects, advantages, features and characteristics of the presentinvention will become more apparent upon consideration of the followingdescription and the appended claims.

SUMMARY OF THE INVENTION

The present invention provides a powder coating composition that willnot readily agglomerate during storage and can be cured at temperaturesas low as about 160° C. without the use of an urethane catalyst, and attemperatures less than about 160° C. with the use of urethane catalyst.Moreover, the present invention has an additional advantage of utilizingcrosslinking agents, which when unblocked, do not release blockingagents into the environment.

The powder coating composition of the invention comprises a uniquecombination of a branched oligoester polyol and crosslinking agent whichwhen cured results in a coating with desirable hardness, flexibility,solvent resistance, corrosion resistance, weatherability and gloss. Thebranched oligoester polyol has a unique combination of branchedstructure, number average molecular weight, hydroxyl number, and acidnumber which provides a relatively high glass transition temperature,and hence, agglomeration resistance. When the latter branched oligoesterpolyol is cured with an uretdione, the combination of branchedoligoester polyol and uretdione provides a coating with good performancecharacteristics without the production of volatile organic compounds(VOCs) with or without the use of urethane catalysts. The inventionprovides an increase in reactivity and high rate of cure at lowertemperatures without VOCs and without sacrificing storage stabilitybecause of agglomeration or sintering.

The branched oligoester polyol has a Tg of at least about 40° C. toabout 80° C., a number average molecular weight of from about 1000 toabout 7500 daltons, a hydroxyl functionality of about 1.5 to about 5.0,a hydroxyl number of from about 15 to about 250 and an acid number ofabout 1 to about 25, and in an very important aspect, an acid number ofabout 5 to about 7. In another important aspect, the branched oligoesterwill have a viscosity of from about 20 to about 90 poise at about 200°C.

The powder coating composition of the invention comprises the branchedoligoester polyol and uretdione powder coating crosslinking agent eachin relative amounts which are effective for providing crosslinkedcoating compositions with a pencil hardness of at least about HB, adirect impact resistance of at least about 80 in lb and a reverse impactresistance of at least about 80 in lb at a binder thickness of about 0.8to about 4 mils when curing is conducted at temperatures below about160° C. and up to about 350° C. The powder coating composition of theinvention which comprises the branched oligoester polyol and uretdionehas a Tg of from about 40° C. to about 80° C. In an important aspect thepowder coating composition comprises from about 40 to about 97 weightpercent of the branched hydroxyl terminated oligoester, based on theweight of branched oligoester polyol and crosslinking agent.

The branched oligoester polyol may be synthesized by forming a generallylinear hydroxyl terminated oligoester diol by reacting a diol and adiacid and then reacting the resulting hydroxyl terminated oligoesterdiol with less than a stoichiometric amount (relative to the hydroxylson the oligoester) of a polyacid having a carboxyl functionality of atleast about 3. This less than stoichiometric amount provides somecarboxyl groups to the oligomer, but its more important purpose is togenerally provide complex branching of the oligoester polyol so thatoligomer chains extend in some cases, from all of the carboxylfunctionality of the polyacid and some of the polyacids areinterconnected by oligomer chains. In an important aspect, the carboxylfunctionality from the polyacid reacted with the oligoester is not morethan about 15% of the equivalents of the stoichiometric amount ofcarboxyl equivalent needed to react with all of the hydroxyl groups ofthe oligoester. In an important aspect, the ratio of hydroxyl terminatedoligoester diol to triacid is from about 9.0:1 to about 30:1, preferablyabout 10:1 to about 20:1.

In an important aspect of the invention, the hydroxyl terminated diol isthe reaction product of an aliphatic diol (open chain or cycloaliphatic)and an aromatic diacid, diacid halide, or diacid anhydride, such asterephthalic acid, which provides a hydroxyl terminated oligoester diolhaving aromatic groups. Alternatively in this aspect, the acid may be astraight chain or cycloaliphatic diacid, diacid anhydride or diacidhalide, and the diol may be hydroquinone to provide the oligoester witharomatic monomers along its main chain.

In another important aspect, the diol used for the oligoester diol is astraight chain aliphatic or cycloaliphatic diol and the diacid is acycloaliphatic diacid, diacid anhydride, or diacid halide, whichmonomers provide an oligoester diol having cycloaliphatic groups.

In yet another aspect, if the diacid, diacid anhydride or halide anddiol used to make the oligoester diol are both straight chain, anaromatic monomer having hydroxyl and carboxyl functionality may be usedto improve properties of the ultimate coating composition. Thesearomatic monomers having hydroxy and carboxyl functionality includeortho, meta, and parahydroxybenzoic acid. While not intending to bebound by any theory, it appears that the ring and straight chaincombination, or aromatic cycloaliphatic combination provide desired filmproperties.

The hydroxyl terminated oligoester diol is the reaction product ofexcess diol with a diacid. The diol may be one or more diols selectedfrom the group consisting of neopentyl glycol, 1,6 hexane diol,2-butyl-2-ethyl-1,3-propane diol, 1,4 cyclohexanedimethanol, diethyleneglycol, 1,3 propanediol, hydrogenated bisphenol A,2,3,4,4-tetramethyl-1,3-cyclobutanediol, ethylene glycol, propyleneglycol, 2,4-dimethyl-2-ethylhexane-1,3-diol,2-ethyl-2-isobutyl-1,3-propanediol, 1,3-butanediol, 1,4-butanediol,1,5-pentanediol, thiodiethanol, 1,2-cyclohexanedimethanol,1,3-cyclohexanedimethanol, 1,4-xylylenediol, ethoxylated bisphenol A,ester diol 204 (Union Carbide), 3-hydroxy-2,2-dimethylproprionate,unoxol 6 diol, methyl propanediol, 2-methyl-1,3-propane diol,hydroxypivalyl hydroxypivalate (HPHP), vinyl cyclohexanediol,dipropylene glycol, ester diols, dimethylol proprionic acid (DMPA), andmixtures thereof.

The aromatic acids/anhydrides/acid halides used in the invention areselected from the group consisting of terephthalic acid, phthalic acid,phthalic anhydride, dimethyl terephthalic acid, naphthalenedicarboxylate, tetrachlorophthalic acid, terephthalic acid bisglycolester, isophthalic acid, t-butyl isophthalic acid, and mixtures thereof.

Aliphatic acids/anhydrides/acid halides useful in the present inventionare selected from the group consisting of fumaric acid, adipic acid,azelaic acid, sebacic acid, dodecanoic acid, glutaric acid, succinicacid, oxalic acid, itaconic acid, dimer fatty acids, maleic anhydride,succinic anhydride, chlorendic acid, diglycolic acid, nadic acid, andmixtures thereof.

Cycloaliphatic acids/anhydrides/acid halides used in the invention mayinclude acids/anhydrides such as 1,4-cyclohexane diacid, 1,3-cyclohexanedicarboxylic acid, hexahydrophthalic anhydride, dimethyl cyclohexanedicarboxylate, and mixtures thereof. Mixtures of these compounds mayalso be used for the preparation of the ester diols.

Aromatic diols or dihydroxy phenolic compounds which may be used to makethe oligoester diol include hydroquinone, catechol, resorcinol,p,p′-dihydroxy diphenyl methane, bisphenol A, p,p′-dihydroxy diphenylketone, p,p′-dihydroxydiphenyl, and mixtures thereof. Typically whensuch phenolic type dihydroxy compounds are used to make the oligoesterdiols, base catalysis is typically required.

The reaction which forms the hydroxyl terminated oligoester diol isconducted for a time and temperature effective to provide an oligoesterdiol having a number average molecular weight in the range of from about400 to about 1500 daltons and then the reaction is slowed by cooling toabout 170° C. to about 200° C. to provide the latter oligoester diol.Generally, the reaction which provides the oligoester diol is conductedat a temperature of about 240° C. for about 4 to about 15 hours beforethe reaction is cooled.

This relatively low molecular weight hydroxyl terminated oligoester diolthen is reacted with the polyacid/anhydride/polyols or mixtures thereofselected from citric acid, pyromellitic anhydride, trimelliticanhydride, trimethylolpropane, trimethyolethane, pentaerythritol, andditrimethyolpropane. In an important aspect of the invention, thepolyacid or triacid which is reacted with the hydroxyl terminatedoligoester diol is an aromatic acid. The branching reaction is conductedfor a time and temperature effective for providing the branchedoligoester polyol described herein. The reaction which forms thebranched oligoester polyol is conducted at a temperature of about 180°C. to about 240° C. for about 4 to about 15 hours.

Uretdione is an important crosslinking agent in the present invention.The amount of crosslinking agent in the composition is effective forproviding an equivalent ratio of isocyanate groups to hydroxyl groups offrom about 0.5:1 to about 1.8:1.

In an important aspect of the invention, the uretdione has the followingstructure

where R can include compounds from the monomeric diisocyanates such as4,4′-diisocyanatodicyclohexylmethane, 1,4-diisocyanatobutane,1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane, 1,3- and1,4-phenylene diisocyanate, naphthylene-1,5-diisocyanate, 2,4- and/or2,6-toluylene diisocyanate, diphenylmethane-2,4′- and/or4,4′-diisocyanate, 1,3- and 1,4-diisocyanatocyclohexane,1,6-diisocyanatohexane, 1,10-diisocyanatodecane, 2,2,4-and2,4,4-trimethyl-1,6-diisocyanatohexane,1,5-diisocyanato-2,2-dimethylpentane, and others known in the art.

The uretdione structure is a result of the catalytic dimerization ofmonomeric diisocyanates, which are separated from the unreactedexcess-isocyanate monomer in the presence of a catalyst. The unreactedisocyanate groups of the uretdione structure are then chain extendedwith ester diols. A typical uretdione structure may be the dimerizationproduct of 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethyl cyclohexane(isophorone diisocyante; IPDI). Typical functionality of these oligomersis approximately 2. The known uretdione dimer is thermally cleaved inthe presence of a catalyst at lower temperatures to yield a cured powdercoating at a low temperature bake schedule. Uncatalyzed uretdione dimerstypically thermally cleave at approximately 160° C. to further reactwith the hydroxyl groups of the branched oligoester polyol in forming acured powder coating composition. Catalyzed uretdione dimers in a powdercoating thermally cleave at temperatures less than 160° C. to furtherreact with the hydroxyl groups of the branched oligoester polyol informing a cured powder coating composition.

Cleavage of the uretdione ring in the presence of hydroxyl functionalreactants is initiated at temperatures as low as about 160° C. andcuring of the powder coating composition proceeds without the need foran effective amount of polyurethane catalysts. Generally, less thanabout 0.02 weight percent urethane catalyst, based on the weight of thepowder coating composition, is not effective for urethane catalysis anddoes not increase reaction rates. The uretdione, when mixed withbranched oligoester polyol and cured, does not emit any volatiles as theuretdione ring opens to generate isocyanate (NCO) groups to crosslinkwith hydroxyl groups of the branched oligoester to give cured powdercoatings. In an important aspect, when uretdione is used as thecrosslinking agent in the composition, the composition contains fromabout 3 to about 60 weight percent uretdione powder coating crosslinkingagent, based on the weight of branched oligoester polyol andcrosslinking agent.

In another important aspect, the present invention further provides aprocess for the preparation of powder coating compositions wherein thebranched oligoester polyol prepared as described herein, is blended witha uretdione powder coating crosslinking agent, and optionally withauxiliary substances conventionally used in the manufacture of powdercoatings.

DETAILED DESCRIPTION OF THE INVENTION

Definitions

As used herein “coating binder” is the polymeric portion of a coatingfilm after baking and after crosslinking.

“Polymeric vehicle” means all polymeric and resinous components in theformulated coating; i.e. before film formation. Pigments and additivesmay be mixed with the polymeric vehicle to provide a formulated powdercoating composition.

“Diol” is a compound with two hydroxyl groups. “Polyol” is a compoundwith two or more hydroxyl groups.

“Diacid” is a compound with two carboxyl groups. “Polyacid” is acompound with two or more carboxyl groups and may be an acid or acidanhydride.

A “film” is formed by application of the powder coating composition to abase or substrate, and subsequent crosslinking.

“Sintering” means the loss of particulate characteristics of the powderduring storage resulting in lumps and agglomeration or, in extremecases, a solid mass. Amounts of material are used in -the composition ofthe present invention which are effective for providing a powderedcoating that is substantially nonsintered. “Substantially non sintered”means that after exposure of a powder to a given set of conditions,after cooling, to room temperature, it retains its particulatecharacteristics with only a few lumps which can be readily broken upwith moderate pressure.

A “catalyst” is defined as an additive to a resin/curative or powdercoating composition at a desired concentration level which acceleratesthe chemical reaction at a prescribed temperature and pressure. The typeof catalyst used may be described as a gas, liquid, and/or solid. Asolid catalyst may be mounted on a support/carrier to provide a definedconcentration level per weight of catalyst for a prescribed level ofactivity. The catalysts described are not limited only toesterification, transesterification, and urethane cure chemistries.

“Polyester” means a polymer which has

linkages in the main chain of the polymer. “Oligomer” means a compoundthat generally has repeating monomeric units and is similar to apolymer, but has a number average weight not greater than about 7500daltons with or without repeating monomeric units. A “polymer” will havea number average molecular weight of over about 7500 daltons.

Acid number or acid value means the number of milligrams of potassiumhydroxide required for neutralization of free acids present in 1 g ofresin.

“Hydroxyl number” or “hydroxyl value” which is also called “acetylvalue” is a number which indicates the extent to which a substance maybe acetylated; it is the number of milligrams of potassium hydroxiderequired for neutralization of the acetic acid liberated on saponifying1 g of acetylated sample.

Branched Hydroxyl Terminated Oligoester Resin

Both the Tg and melt viscosity of the resin are greatly influenced bythe choice of monomers. In an important aspect of the invention, thebranched hydroxyl terminated oligoester resin is made by a two stageprocess. In stage one, a hydroxyl terminated oligoester diol isprepared, and in stage two a branched hydroxyl terminated oligoesterpolyol is formed.

Stage One: In stage one, a hydroxyl terminated oligoester diol is formedthrough the esterification or condensation reaction of a stoichiometricmolar excess of a diol (relative to the carboxyls on the acid) with adicarboxylic acid, dicarboxylic acid anhydride or dicarboxylic acidhalide such as an acid chloride.

(1) Diols which may be used in the reaction may be selected from thegroup consisting of neopentyl glycol, 1,6 hexane diol,2-butyl-2-ethyl-1,3-propane diol, 1,4 cyclohexanedimethanol, diethyleneglycol, 1,3 propanediol, hydrogenated bisphenol A,2,3,4,4-tetramethyl-1,3-cyclobutanediol, ethylene glycol, propyleneglycol, 2,4-dimethyl-2-ethylhexane-1,3-diol,2-ethyl-2-isobutyl-1,3-propanediol, 1,3-butanediol, 1,4-butanediol,1,5-pentanediol, thiodiethanol, 1,2-cyclohexanedimethanol,1,3-cyclohexanedimethanol, 1,4-xylylenediol, ethoxylated bisphenol A,ester diol 204 (Union Carbide), 3-hydroxy-2,2-dimethylproprionate,unoxol 6 diol, methyl propanediol, 2-methyl-1,3-propane diol,hydroxypivalyl hydroxypivalate (HPHP), vinyl cyclohexanediol,dipropylene glycol, ester diols, dimethylol proprionic acid (DMPA), andmixtures thereof.

Aromatic diols such as hydroquinone, catechol, resorcinol,p,p′-dihydroxy diphenyl methane, bisphenol A, p,p′-dihydroxy diphenylketone, p,p′-dihydroxydiphenyl, and mixtures thereof, also may bereacted with straight chain or cycloaliphatic diacids.

(2) Aromatic diacids, aliphatic diacids and/or cycloaliphatic diacids oranhydrides or acid halides may be used to make the hydroxyl terminateddiols.

In an important aspect, the aromatic acid/anhydride/acid halide isselected from the group consisting of terephthalic acid, phthalic acid,phthalic anhydride, dimethyl terephthalic acid, naphthalenedicarboxylate, tetrachlorophthalic acid, terephthalic acid bisglycolester, isophthalic acid, t-butyl isophthalic acid, and mixtures thereof,or acid halides thereof.

Aliphatic acid/anhydrides/acid halides which may be used in theinvention include fumaric acid, adipic acid, azelaic acid, sebacic acid,dodecanoic acid, glutaric acid, succinic acid, oxalic acid, itaconicacid, dimer fatty acids, maleic anhydride, succinic anhydride,chlorendic acid, diglycolic acid, nadic acid, and mixtures thereof.

Cycloaliphatic acid/anhydrides/acid halides which may be used in theinvention include acids/anhydrides such as 1,4-cyclohexane diacid,1,3-cyclohexane dicarboxylic acid, hexahydrophthalic anhydride, dimethylcyclohexane dicarboxylate, and mixtures thereof.

Mixtures of these compounds may also be used for the preparation of theester diols as mixed functional intermediates. Ester diols are thoseprepared in a known manner from lactones and dihydric alcohols asstarter molecules through a ring opening reaction. The preparation ofester diols may include lactones such as β-propiolactone,γ-butyrolactone, γ- and delta-valerolactone, ε-caprolactone, 3,5,5,- and3,3,5-trimethylcaprolactone or mixtures thereof. Suitable startermolecules include the described dihydric alcohols listed.

In a very important aspect of the invention, the aromatic acid is one ormore aromatic acids selected from the group consisting of terephthalicacid (TPA), isophthalic acid (IPA), and t-butyl isophthalic acid. Theseare reacted with an aliphatic or cycloaliphatic diol such as neopentylglycol, 1,6 hexane diol, 2-butyl-2-ethyl, 1,3-propanediol, and1,4-cyclohexane dimethanol.

The diol component and diacid component are each present in amountseffective for providing the coating composition and subsequent coatingwith the properties described. In an important aspect of the invention,the combination of neopentyl glycol and 1,6 hexane diol in a molar ratioof about 4.0:1 to about 7.0:1, preferably about 5.6:1, is reacted withTPA, IPA or t-butyl isophthalic acid provides a coating composition withan acceptable Tg.

The reaction to obtain hydroxyl terminated oligoester diol may beconducted at about 240° C. for about 4 to about 15 hours and then iscooled to obtain the generally linear product having the Mn of fromabout 400 to about 1500 daltons. If an aliphatic acid is reacted with anaromatic dihydroxyl compound such as hydroquinone, catechol, resorcinol,p,p′-dihydroxy diphenyl methane, bisphenol A, p,p′-dihydroxy diphenylketone, p,p′-dihydroxydiphenyl, and mixtures thereof, the use of basecatalysts typically are required.

Stage Two: In stage two, the hydroxyl terminated oligoester diolprepared in stage one is reacted to form a branched hydroxyl terminatedoligoester polyol. In an important aspect of the invention, the hydroxylterminated oligoester prepared in stage one is reacted with apolyacid/anhydrides or blend of polyacids/anhydrides which are at leasta triacid. The triacid, or triacid blend is selected from the groupconsisting of trimellitic anhydride (TMA) and citric acid. In a veryimportant aspect, the triacid is an aromatic acid such as trimelliticanhydride.

In an important aspect of the invention, the resulting branched hydroxylterminated oligoester has a hydroxyl functionality of about 1.5 to about5.0, a hydroxyl number of from about 15 to about 250, an acid value ofabout 1 to about 25, and a number average molecular weight in the rangeof from about 1000 to about 7500 daltons. The branched hydroxylterminated oligoester has a Tg of at least about 40° C., and in animportant aspect from about 40° C. to about 80° C. In a very importantaspect, the ratio of hydroxyl terminated oligoester to polyacid is about9.0:1 to about 30:1.

Crosslinking Agents

Uretdione Crosslinkers: In an important aspect of the invention, theuretdione crosslinker is an internally blocked isocyanate or a dimer ofan isocyanate. Examples of uretdione crosslinkers acceptable for use inthe present invention include Crelan LS2147 (Bayer), and Alcure 4147(McWhorter Technologies). The preparation of uretdione crosslinkingagent provides an average NCO functionality, based on the free NCOgroups, of about 1.9. The free NCO content is typically less than about1%. In a very important aspect, the powder coating composition willinclude about 3 to about 60 weight percent uretdione crosslinking agent,based on the weight of the branched hydroxyl terminated oligoesterpolyol and crosslinking agent.

Preparation and Application of the Thermosetting Powder

For the preparation of the thermosetting powder compositions, thebranched hydroxyl terminated oligoester resin, the crosslinking agentand various auxiliary substances conventionally used for the manufactureof powder coatings and paints are mixed homogeneously. Thishomogenization is carried out for example by melting the oligoester, thecrosslinking agent and the various auxiliary substances at a temperaturewithin the range of from about 70° to about 130° C., preferably in anextruder, for example a Buss-Ko-Kneader extruder or a twin-screwextruder of the Werner-Pfleiderer or Baker Perkins type. The extrudateis then allowed to cool, is ground and sieved to obtain a powdersuitable for electrostatic or fluidized bed application.

Another factor affecting viscosity and flow is the level of pigmentationand fillers in the system. High levels of pigmentation and/or fillersdetract from the flow of the system by increasing the melt viscosity.Fine particle size organic pigments such as carbon black, phthalocyanineblue and quinacridones cause a significant increase in melt viscosityeven at low levels.

The auxiliary substances which can be added to the thermosetting powdercompositions according to the invention include ultraviolet lightabsorbing compounds such as Tinuvin 900 (from CIBA-GEIGY Corp.), lightstabilizers based on sterically hindered amines (for example Tinuvin 144from CIBA-GEIGY Corp.), phenolic antioxidants (for example Irganox 1010and Irgafos from CIBA-GEIGY Corp.) and stabilizers of the phosphonite orphosphite type. A variety of pigments may also be added to thethermosetting powder compositions according to the invention. Examplesof pigments that may be employed in the invention are metal oxides suchas titanium dioxide, iron oxide, zinc oxide and the like, metalhydroxides, metal powders, sulfides, sulfates, carbonates, carbon black,iron blues, organic reds, organic yellows, organic maroons and the like.Auxiliary substances may also include flow control agents such asResiflow PV5 (from WORLEE), Modaflow 3 and 2000 (from MONSANTO), Acronal4F (from BASF), Resiflow P-67 (from Estron), plasticizers such asdicyclohexyl phthalate, triphenyl phosphate, grinding aids and degassingagents such as benzoin. Examples of fillers are calcium carbonate,magnesium carbonate, blanc fixe, barytes, silicates, talc, china clayand the like. These auxiliary substances are added in conventionalamounts, it being understood that if the thermosetting powdercompositions of the inventions are used as clear coatings, opacifyingauxiliary substances should be omitted.

In addition, urethane catalysts can also be mixed with the thermosettingpowder composition of the invention. Catalysts useful in the presentinvention include 1,5-diazabicyclo(4.3.0)non-5-ene,1,8-diazabicyclo(5.4.0)undec-7-ene, dibutyltin dilaurate, butanestannoic acid, dibutyltin oxide, stannous oxide, and others known in theart.

The powder coating compositions which are the subject matter of thepresent invention are suitable to be applied on articles to be coatedby, but not limited to conventional techniques, e.g. by application bymeans of an electrostatic or tribostatic spray gun; Powder Cloudtechnology (Material Sciences Corporation) or by the well-knownfluidized bed coating technique. In an important aspect, the compositionof the present invention can be used to supply very thick coatings.

After having been applied on the article in question, the depositedcoatings are cured by heating in an oven. In an important aspect, curingwithout catalyst is effected at a temperature of as low as about 160° C.for about less that 35 minutes in order to obtain sufficientcrosslinking to provide the described coating properties. Alternatively,desirable coating properties can be obtained by curing at a temperatureof about 200° C. for about 5 minutes, heating at about 180° C. for about15 minutes, and coil coating by heating at about 350° C. for about 40seconds. Curing by infrared (IR) or induction heating is alsoapplicable. In the aspect of the invention where urethane catalyst isused, desirable coating properties can be obtained by curing at atemperature as low as about 130° C. for about 10 to about 30 minutes.

The following examples illustrate methods for carrying out the inventionand should be understood to be illustrative of, but not limiting upon,the scope of the invention which is defined in the appended claims.

EXAMPLES Example 1 Stage 1

Preparation of hydroxyl terminated oligomer Reactant Weight NeopentylGlycol (Eastman) 805 gms 1,6 hexanediol (UBE) 161 gms Terephthalic acid(Amoco) 1145 gms Butylchlorotin dihydroxide 2.1 gms (Elf-Atochem)Antioxidant 4.6 gms (Weston 618-General Electric)

The mixture was heated gradually to 205° C. and then processed at 240°C. to an acid value of 4 to 7 mg KOH/gram resin with an ICI cone andplate viscosity of 10-14 poise at 125° C. The hydroxyl number of thisoligomer was found to be about 130-140 mg KOH/gram resin.

Stage 2

Preparation of hydroxyl terminated Branched Oligoester

The above oligomer was cooled to 180° C. and 153.5 gms of trimelliticanhydride were added. The temperature was raised to 210-215° C. andvacuum was slowly applied over a 50 minute period until a vacuum of23-24 inches of mercury was obtained. The reaction was monitored byregularly taking a sample and determining the acid number and ICI coneand plate viscosity @ 200° C. When an ICI cone and plate viscosity of50-60 poise and an acid value of 4-10 mg KOH/gram resin were obtained,the melt was cooled down to 195° C. and discharged from the flask. Thecolor of the resin was nearly colorless/transparent to light yellow incolor. Further examples of the hydroxyl terminated branched oligoesterare shown in table 1.

Example 2

Preparation of Powder Coatings All oligoesters were made into a highgloss, white powder coating prepared as follows Oligoester Resin ofExample 1 1000 grams Alcure 4147 (McWhorter Technologies) 310 gramsModaflow 2000 15 grams (flow leveling agent-Monsanto) Benzoin (degassingagent) 6 grams Titanium Dioxide (Dupont R-960) 450 grams

All the above ingredients are initially mixed in a high speed mill suchas a Welex mixer, where a homogeneous mix is obtained. The resulting mixwas processed through a twin screw extruder (ZSK 30 Werner-Phleider)with Zone 1 at 80° C. and Zone 2 at 122° C. The resulting melt wasdischarged onto a pair of water cooled squeeze rolls, from which theemerging cooled sheet was roughly crushed prior to pulverizing in aBrinkman grinding mill. The resulting powder was sieved through a 100mesh screen. This powder coating was electrostatically sprayed ontoground steel panels. The physical properties of the formulated powdercoating are determined after a 5 minute cure at 200° C. for 1.5-2.2 milsfilm thickness. The composition and the test results of these powdercoatings are given in Table 2.

TABLE 1 Composition and Properties of the Oligoesters Composition(grams) 1 2 3 4 5 6 7 Neopentyl Glycol 953 881 881 878 809 806 804 1.6Hexane diol — 80 80 80 161 160 160 Terephthalic Acid 1145 1144 1144 11401145 1138 1135 Butylchlorotin Dihydroxide 2.26 2.26 2.26 2.26 2.26 2.262.26 Weston 618 4.52 4.52 4.52 4.52 4.52 4.52 4.52 Trimelletic Anhydride161.00 163.00 163.00 171.00 155.00 163.00 171.00 Total 2265.78 2274.782274.78 2275.78 2276.78 2273.78 2276.78 —H₂O 265.78 274.78 274.78 275.78276.78 273.78 276.78 Yield 2000.00 2000.00 2000.00 2000.00 2000.002000.00 2000.00 RESIN PROPERTIES Acid Number 10.1 8.3 7.2 9.5 5.2 7.49.3 Hydroxyl Number 60 55 62 65 64 64 64 ICI Visc @ 200° C. 55 62 62 5752 55 61 Glass Transition Temp ° C. 66 59 60 59 51 53 52 Number Av. Mol.Wt 2950 3140 3666 3190 3123 3179 3042 (GPC)

TABLE 2 Film Evaluation of the Oligoesters 30-3000 CompetitiveCompetitive Resin 1 2 3 4 5 6 7 Control Resin 1 Resin 2 Gloss 60° 92 9290 92 90 89 90 90 91 88 20° 82 80 72 76 77 75 76 76 78 66 ImpactResistance in lb Direct 160 160 160 160 160 160 160 80 160 160 Reverse160 160 160 160 160 160 160 10 160 160 MEK Rubs* 4 4 4 3 3.5 4 4 3 3.5 3(50 double rubs) Flow/Leveling** 6 6 6 6 6 6 6 7 4 2 Gel time 89 90 8377 94 80 77 92 98 47 400° F. secs 45° Inclined Plate 150 148 126 147 145139 126 150 150 98 Pill Flow @ 375° F. mm *Judged visually on a scale of0-5 in which 0 = very bad 5 = excellent **PCI flow standards on a scaleof 1-10 in which 1 = poor flow 10 = Smooth

Numerous modifications and variations in practice of the invention areexpected to occur to those skilled in the art upon consideration of theforegoing detailed description of the invention. Consequently, suchmodifications and variations are intended to be included within thescope of the following claims.

What is claimed is:
 1. A branched hydroxyl terminated oligoester polyolcomprising the reaction product of a hydroxyl terminated oligoester diolhaving a number average molecular weight in the range of from about 400to about 1500 daltons and a polyacid having a carboxyl functionality ofabout 3 or greater, the branched hydroxyl terminated oligoester polyolhaving a hydroxyl value in the range of from about 15 to about 250, anacid number of about 1 to about 25, and a number average molecularweight in the range of from about 1500 to about 7500 daltons, whereinthe hydroxyl terminated oligoester diol is the reaction product of adiol reactant and a diacid reactant, the diol reactant selected from thegroup consisting of neopentyl glycol, 1,6 hexane diol,2-butyl-2-ethyl-1,3-propane diol, 1,4 cyclohexanedimethanol, diethyleneglycol, 1,3 propanediol, hydrogenated bisphenol A, dimethyol proprionicacid, 2,3,4,4-tetramethyl-1,3-cyclobutanediol, ethylene glycol,propylene glycol, 2,4-dimethyl-2-ethylhexane-1,3-diol,2-ethyl-2-isobutyl-1,3-propanediol, 1,3-butanediol, 1,4-butanediol,1,5-pentanediol, thiodiethanol, 1,2-cyclohexanedimethanol,1,3-cyclohexanedimethanol, 1,4-xylylenediol, ethoxylated bisphenol A,methyl propanediol, 2-methyl-1,3-propane diol, hydroxypivalylhydroxypivalate, vinyl cyclohexanediol, dipropylene glycol, dimethylolpropionic acid, aromatic diol compounds and mixtures thereof, the diacidreactant selected from the group consisting of an open chain aliphaticdicarboxylic acid, a cycloaliphatic dicarboxylic acid, an aromaticdicarboxylic acid, an open chain aliphatic dicarboxylic acid anhydride,a cycloaliphatic dicarboxylic acid anhydride, an aromatic dicarboxylicacid anhydride, an open chain dicarboxylic acid halide, a cycloaliphaticdicarboxylic acid halide, an aromatic dicarboxylic acid halide andmixtures thereof, wherein the diacid reactant includes a cycloaliphaticdiacid reactant where the cycloaliphatic diacid reactant is thecycloaliphatic acid, the cycloaliphatic anhydride or cycloaliphatic acidhalide of 1,4-cyclohexane dicaboxylic acid, 1,3-cyclohexane dicarboxylicacid, hexahydrophthalic acid dimethyl cyclohexane dicarboxylic acid ormixtures thereof.
 2. A branched hydroxyl terminated oligoester polyol asrecited in claim 1, wherein the diacid reactant includes aromatic diacidreactant and the aromatic diacid reactant is an aromatic acid, acidanhydride or acid halide of terephthalic acid, phthalic acid, phthalicanhydride, dimethyl terephthalic acid, naphthalene dicarboxylate,tetrachlorophthalic acid, terephthalic acid bisglycol ester, isophthalicacid, t-butyl isophthalic acid, or mixtures thereof.
 3. A branchedhydroxyl terminated oligoester polyol as recited in claim 1, wherein thediacid reactant includes an open chain aliphatic diacid reactant wherethe open chain aliphatic diacid reactant is the open chain aliphaticacid, acid anhydride or acid halide of fumaric acid, adipic acid,azelaic acid, sebacic acid, dodecanoic acid, glutaric acid, succinicacid, oxalic acid, itaconic acid, dimer fatty acids, maleic acid,chlorendic acid, diglycolic acid, nadic acid, or mixtures thereof.
 4. Abranched hydroxyl terminated oligoester polyol as recited in claim 1,wherein the diol reactant is selected from the group consisting ofneopentyl glycol, 1,6 hexane diol, 2 -butyl-2-ethyl-1,3-propane diol,1,4 cyclohexanedimethanol, and mixtures thereof, the diacid reactantincludes an aromatic diacid reactant and the aromatic diacid reactant isan aromatic acid, acid anhydride or acid halide of terephthalic acid,phthalic acid, phthalic anhydride, dimethyl terephthalic acid,naphthalene dicarboxylate, tetrachlorophthalic acid, terephthalic acidbisglycol ester, isophthalic acid, t-butyl isophthalic acid, or mixturesthereof.
 5. A branched hydroxyl terminated oligoester polyol as recitedin claim 1, wherein the diol reactant is the reaction product of alactone selected from the group consisting of β-propiolactone,γ-butyrolactone, γ- and delta-valerolactone, ε-caprolactone, 3,5,5,- and3,3,5-trimethylcaprolactone and mixtures thereof, and a dihydriccompound.
 6. A branched hydroxyl terminated oligoester polyol as recitedin claims 1, 2, 3, or 4 wherein the polyacid is a triacid.
 7. A branchedhydroxyl terminated oligoester polyol as recited in claim 6, wherein thetriacid is selected from the group consisting of trimellitic anhydride,citric acid, and mixtures thereof.
 8. A branched hydroxyl terminatedoligoester polyol as recited in claim 7, wherein the triacid istrimellitic anhydride.
 9. A branched hydroxyl terminated oligoesterpolyol as recited in claim 1, wherein the hydroxyl terminated oligoesterdiol is the reaction product of a hydroxyl functional ester compound anda diacid reactant selected from the group consisting of an open chainaliphatic dicarboxylic acid, a cycloaliphatic dicarboxylic acid, anaromatic dicarboxylic acid, an open chain aliphatic dicarboxylic acidanhydride, a cycloaliphatic dicarboxylic acid anhydride, an aromaticdicarboxylic acid anhydride, an open chain dicarboxylic acid halide, acycloaliphatic dicarboxylic acid halide, an aromatic dicarboxylic acidhalide and mixtures thereof.
 10. A branched hydroxyl terminatedoligoester polyol as recited in claim 9, wherein the hydroxyl functionalester compound is 3-hydroxy-2,2-dimethylproprionate.
 11. A branchedhydroxyl terminated oligoester polyol as recited in claim 10, whereinthe diacid reactant is an aromatic diacid reactant and the aromaticdiacid reactant is an aromatic acid, acid anhydride or acid halide ofterephthalic acid, phthalic acid, phthalic anhydride, dimethylterephthalic acid, naphthalene dicarboxylate, tetrachlorophthalic acid,terephthalic acid bisglycol ester, isophthalic acid, t-butyl isophthalicacid, or mixtures thereof.
 12. A process for preparing a branchedhydroxyl terminated oligoester polyol, the process comprising: blendinga hydroxyl terminated oligoester diol and a polyacid having a carboxylfunctionality of about 3 or greater, wherein the equivalent ratio ofhydroxyl terminated oligoester diol to polyacid is from about 9:1 toabout 30:1; and reacting the hydroxy terminated diol and the polyacidfor a time and temperature effective to provide the branched hydroxylterminated oligoester polyol which has a hydroxyl value in the range offrom about 15 to about 250, an acid number of about 1 to about 25, and anumber average molecular weight in the range of from about 1500 to about7500 daltons, the hydroxy terminated oligoester diol being the reactionproduct of a diol reactant and a diacid reactant selected from the groupconsisting of an open chain aliphatic dicarboxylic acid, acycloaliphatic dicarboxylic acid, an aromatic dicarboxylic acid, an openchain aliphatic dicarboxylic acid anhydride, a cycloaliphaticdicarboxylic acid anhydride, an aromatic dicarboxylic acid anhydride, anopen chain dicarboxylic acid halide, a cycloaliphatic dicarboxylic acidhalide, an aromatic dicarboxylic acid halide and mixtures thereof, thediol reactant and diacid reactant being reacted for a time andtemperature to provide a hydroxy terminated diol with a number averagemolecular weight in the range of from about 400 to about 1500 daltons,cooling the reaction of the diol reactant and diacid reactant at aboutthe time the oligoester diol reaches the number average molecular weightin the range of 400 to about 1500 and before reacting the hydroxyterminated diol with the polyacid, wherein the diacid reactant includesa cycloaliphatic diacid reactant where the cycloaliphatic diacidreactant is the cycloaliphatic acid, the cycloaliphatic anhydride orcycloaliphatic acid halide of 1,4-cyclohexane dicaboxylic acid,1,3-cyclohexane dicarboxylic acid, hexahydrophthalic acid, dimethylcyclohexane dicarboxylic acid or mixtures thereof.
 13. A process forpreparing a branched hydroxyl terminated oligoester polyol as recited inclaim 12, wherein the diacid reactant includes an aromatic diacidreactant where the aromatic diacid reactant is the aromatic acid, acidanhydride or acid halide of terephthalic acid, phthalic acid, phthalicanhydride, dimethyl terephthalic acid, naphthalene dicarboxylate,tetrachlorophthalic acid, terephthalic acid bisglycol ester, isophthalicacid, t-butyl isophthalic acid, or mixtures thereof.
 14. A process forpreparing a branched hydroxyl terminated oligoester polyol as recited inclaim 12, wherein the dial reactant is selected from the groupconsisting of neopentyl glycol, 1,6 hexane dial,2-butyl-2-ethyl-1,3-propane diol, 1,4 cyclohexanedimethanol, andmixtures thereof, the diacid reactant includes an aromatic diacidreactant and the aromatic diacid reactant is an aromatic acid, acidanhydride or acid halide of terephthalic acid, phthalic acid, phthalicanhydride, dimethyl terephthalic acid, naphthalene dicarboxylate,tetrachlorophthalic acid, terephthalic acid bisglycol ester, isophthalicacid, t-butyl isophthalic acid, or mixtures thereof.
 15. A process forpreparing a branched hydroxyl terminated oligoester polyol as recited inclaim 12, wherein the polyacid is a triacid.
 16. A process for preparinga branched hydroxyl terminated oligoester polyol as recited in claim 15,wherein the branched hydroxyl terminated oligoester polyol has ahydroxyl functionality of about 1.5 to about 5.0.
 17. A process forpreparing a branched hydroxyl terminated oligoester polyol as recited inclaims 12 or 14 wherein the reaction between the diol reactant anddiacid reactant that forms the hydroxyl terminated oligoester diol iscooled to about 170° C. to about 200° C.
 18. A process for preparing abranched hydroxyl terminated oligoester polyol as recited in claims 12or 15 wherein the reaction between the hydroxyl terminated oligoesterdiol and polyacid that forms the branched hydroxyl terminated oligoesterpolyol is conducted at a temperature of about 180° C. to about 240° C.for about 4 to about 15 hours.
 19. A branched hydroxyl terminatedoligoester polyol comprising the reaction product of a hydroxylterminated oligoester dial having a number average molecular weight inthe range of from about 400 to about 1500 daltons and a polyacid havinga carboxyl functionality of about 3 or greater, the branched hydroxylterminated oligoester polyol having a hydroxyl value in the range offrom about 15 to about 250, an acid number of about 1 to about 25, and anumber average molecular weight in the range of from about 1500 to about7500 daltons, wherein the hydroxyl terminated oligoester diol is thereaction product of a diol reactant and a diacid reactant, wherein thediol reactant is the reaction product of a lactone selected from thegroup consisting of β-propiolactone, γ-butyrolactone, γ- anddelta-valerolactone, ε-caprolactone, 3,5,5,- and3,3,5-trimethylcaprolactone and mixtures thereof, and a dihydriccompound.
 20. A branched hydroxyl terminated oligoester polyolcomprising the reaction product of a hydroxyl terminated oligoester diolhaving a number average molecular weight in the range of from about 400to about 1500 daltons and a polyacid having a carboxyl functionality ofabout 3 or greater, the branched hydroxyl terminated oligoester polyolhaving a hydroxyl value in the range of from about 15 to about 250, anacid number of about 1 to about 25, and a number average molecularweight in the range of from about 1500 to about 7500 daltons, whereinthe hydroxyl terminated oligoester diol is the reaction product of ahydroxyl functional ester compound and a diacid reactant selected fromthe group consisting of an open chain aliphatic dicarboxylic acid, acycloaliphatic dicarboxylic acid, an aromatic dicarboxylic acid, an openchain aliphatic dicarboxylic acid anhydride, a cycloaliphaticdicarboxylic acid anhydride, an aromatic dicarboxylic acid anhydride, anopen chain dicarboxylic acid halide, a cycloaliphatic dicarboxylic acidhalide, an aromatic dicarboxylic acid halide and mixtures thereof,wherein the hydroxyl functional ester compound is3-hydroxy-2,2-dimethylproprionate and the diacid reactant is an aromaticdiacid reactant and the aromatic diacid reactant is an aromatic acid,acid anhydride or acid halide of terephthalic acid, phthalic acid,phthalic anhydride, dimethyl terephthalic acid, naphthalenedicarboxylate, tetrachlorophthalic acid, terephthalic acid bisglycolester, isophthalic acid, t-butyl isophthalic acid, or mixtures thereof.